The present invention relates to a method and apparatus for diagnosing and locating a hearing impairment within a patient, and more particularly for determining the presence, location and extent of viability of certain sensori-neural structures within the cochlea of that patient.
An ear can functionally be described as having two portions--a transducing portion which analyzes and processes the sounds or acoustic vibrations which reach it, thereby causing resultant electrical impulses to be sent to the brain; and a conducting portion which conveys the acoustic vibrations to the transducing portion. The conducting portion of the ear consists generally of the outer and middle ear, and the transducing portion consists generally of the inner ear and its connections to the auditory nerve. When a hearing impairment exists, it may be located in either or both of the perceiving and/or the conducting portions of the ear. When the outer or middle ear is impaired, the hearing impairment is described as conductive and when the inner ear or auditory nerve is impaired, the impairment is described as perceptive or "sensori-neural." If the impairment is conductive, it typically can be surgically corrected or compensated for by an amplification of the acoustic vibrations which are to be heard. Such amplification can be accomplished with conventional hearing aids. If, however, the impairment is perceptive or sensori-neural, conventional hearing aids are generally not effective, and a further determination must be made as to whether the impairment is within the inner ear, the auditory nerve, or both, and to what extent the impairment exists, before the possibility of any treatment can be predicted. The complexity of the inner ear and its connections to the auditory nerve complicate this determination.
When the inner ear is impaired, there is generally a failure within the mechanism responsible for converting the acoustic vibrations into the processed electrical signals which are then transmitted to the brain via the auditory nerve. One component of this mechanism involves tiny receptor cells within the inner ear, called hair cells, which are mechanically activated by the acoustic vibrations conducted through the middle ear. In a manner not precisely understood, the mechanically activated hair cells are caused to release a chemical messenger which travels across the junction or synapse between the haircells and the nerve fibers of the auditory nerve, to activate the nerve fibers of the auditory nerve. The nerve fibers respond by generating electrical action potentials which are then propagated toward the brain.
It is also known that the presence of an electrical signal in the vicinity of the inner ear can cause non-impaired or even partially impaired hair cells within the inner ear to vibrate. These vibrations are able to cause an electrical impulse or action potential to be generated within the auditory nerve, in a sequence analogous to that which occurs in a normally functioning ear, even though these vibrations have not been initiated in the conventional manner. This phenomena has been labelled the cochlear electrophonic effect. (See for example the work done by R. Clark Jones, et al, at Harvard University as published in the Journal of the Acoustic Society of America, Volume 12, October 1940, as well as the graduate theses prepared by Edwin C. Moxon at the Massachusetts Institute of Technology, dated September, 1967 (MS) and June, 1971 (Ph.D).
The auditory nerve fibers can also be directly stimulated by applied electrical signals, which cause these fibers to generate action potentials without going through the chemical interface or synapse between the hair cells and the auditory nerve fibers. In order for this direct stimulation of the auditory nerve to occur, the stimulus electrical signal must either be of an adequate magnitude to directly generate action potentials within the auditory nerve fibers, or alternatively, if the applied signal is subthreshold, there must be a sufficient quantity of signals in close enough time proximity to additively stimulate the auditory nerve fibers. This phenomena is also discussed in the work of Jones et al previously referenced.
Although both the cochlear electrophonic effect and the direct stimulation of the auditory nerve involve auditory stimulation with an electrical signal, they do not provide the information necessary to help the physician diagnose the extent or locality of an impairment within the inner ear.